[3.11.y.z extended stable] Patch "workqueue: make rescuer_thread() empty wq->maydays list before exiting" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    workqueue: make rescuer_thread() empty wq->maydays list before exiting

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From e9f4c4ecff8bdc204d846683c8783af026890f48 Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs at cn.fujitsu.com>
Date: Fri, 18 Apr 2014 11:04:16 -0400
Subject: workqueue: make rescuer_thread() empty wq->maydays list before
 exiting

commit 4d595b866d2c653dc90a492b9973a834eabfa354 upstream.

After a @pwq is scheduled for emergency execution, other workers may
consume the affectd work items before the rescuer gets to them.  This
means that a workqueue many have pwqs queued on @wq->maydays list
while not having any work item pending or in-flight.  If
destroy_workqueue() executes in such condition, the rescuer may exit
without emptying @wq->maydays.

This currently doesn't cause any actual harm.  destroy_workqueue() can
safely destroy all the involved data structures whether @wq->maydays
is populated or not as nobody access the list once the rescuer exits.

However, this is nasty and makes future development difficult.  Let's
update rescuer_thread() so that it empties @wq->maydays after seeing
should_stop to guarantee that the list is empty on rescuer exit.

tj: Updated comment and patch description.

Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj at kernel.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 kernel/workqueue.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a8b0147f3a89..2b48572be9b3 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2369,6 +2369,7 @@ static int rescuer_thread(void *__rescuer)
 	struct worker *rescuer = __rescuer;
 	struct workqueue_struct *wq = rescuer->rescue_wq;
 	struct list_head *scheduled = &rescuer->scheduled;
+	bool should_stop;

 	set_user_nice(current, RESCUER_NICE_LEVEL);

@@ -2380,11 +2381,15 @@ static int rescuer_thread(void *__rescuer)
 repeat:
 	set_current_state(TASK_INTERRUPTIBLE);

-	if (kthread_should_stop()) {
-		__set_current_state(TASK_RUNNING);
-		rescuer->task->flags &= ~PF_WQ_WORKER;
-		return 0;
-	}
+	/*
+	 * By the time the rescuer is requested to stop, the workqueue
+	 * shouldn't have any work pending, but @wq->maydays may still have
+	 * pwq(s) queued.  This can happen by non-rescuer workers consuming
+	 * all the work items before the rescuer got to them.  Go through
+	 * @wq->maydays processing before acting on should_stop so that the
+	 * list is always empty on exit.
+	 */
+	should_stop = kthread_should_stop();

 	/* see whether any pwq is asking for help */
 	spin_lock_irq(&wq_mayday_lock);
@@ -2430,6 +2435,12 @@ repeat:

 	spin_unlock_irq(&wq_mayday_lock);

+	if (should_stop) {
+		__set_current_state(TASK_RUNNING);
+		rescuer->task->flags &= ~PF_WQ_WORKER;
+		return 0;
+	}
+
 	/* rescuers should never participate in concurrency management */
 	WARN_ON_ONCE(!(rescuer->flags & WORKER_NOT_RUNNING));
 	schedule();
--
1.9.1